The long term goal of the work proposed here is to define the functions of homeobox gene products during mouse embryogenesis. Homeobox genes have been suggested to play key roles in establishing the metameric body plan in vertebrates by providing positional cues. We propose to directly test that hypothesis by generating transgenic mouse embryos that inappropriately express homeobox genes and to characterize the resulting phenotypic consequences at both the morphological and biochemical levels. This proposal has two broad but interrelated specific aims: [1] We propose to alter the anterior boundary of expression of a Hox gene during embryonic development and to assess the phenotypic consequences of this inappropriate expression. The promoter regions of two Hox genes, Hox 1.4 and Hox 2.5 will be used to perturb the normal cellular specificity of expression of the Hox 3.1 gene. Both cis-acting control regions will extend the anterior boundary of Hox 3.1 expression in the central nervous system, the mesoderm, or both and will address the importance of those boundaries in normal pattern formation and cytodifferentiation. [2] We propose to disrupt the normal rostro-caudal gradient of expression of Hox 3.1 and Hox 1.4 to examine the function of these gradients in determining the body plan. In these experiments, the coding regions of Hox 3.1 and Hox 1.4 will be driven by Hox 3.1 cis-acting control sequences that fail to be properly down-regulated. The gradients of Hox gene expression will be altered by increasing the level of mRNA accumulation in posterior regions of the embryo. These experiments will provide an insight into the importance of region-specific regulation of the level of homeobox gene expression in the developing embryo. Using these strategies it is anticipated that the normal pattern of Hox 3.1 and Hox 1.4 expression can be perturbed in a controlled manner. Morphologic analysis of the efects of this altered gene expression will provide a basis for defining the role these gene products play in establishing the metameric body plan. We will also examine the effects of altered Hox 3.1 and Hox 1.4 expression on the expression of other homeobox genes to begin to analyze cross-regulatory interactions within a mammalian homeobox gene familuy.